A variant of "that's what she said." Instead of referring to sexual connotations, it is used to refer to spending a lot of money, making a mess, or fucking up very badly. Arose after the 2010 BP oil leak in the Gulf of Mexico.

X: Oh man, I really screwed the pooch on that one.
Y: That's what BP said!

X: It's going to take me all day to clean this mess up.
Y: That's what BP said!